The object of the present invention is a method for the implementation in situ of an anti-corrosive coating made of thermoplastic materials, such as for instance PE/PP (polyethylene/polypropylene), applied onto the welding joints and/or on the damaged areas of the coating of metal pipes used for underground pipelines, and a device for the implementation of said method.
When building underground pipelines (gas-, oil- and water pipelines) metal pipes are normally used, said pipes being externally coated with anti-corrosion thermoplastic materials in suitable areas at the factory or in a suitable yard.
When applying the anticorrosive coating in the plant or in a yard, the ends of said metal tubes are left uncovered (that is to say, they are not provided with the anticorrosive coating) for a certain length, according to the diameter and to the kind of anticorrosive coating, said length being between 50 and 250 mm, so as to be able then to weld the single sections of the tubes, which have been coated in the plant or in a yard, between themselves in situ.
Once the welding of the pipes end has been carried out and checked, it is necessary to coat against corrosion the metal portion which has been left uncovered for the welding and the damaged portions so as to obtain the anticorrosive protection of the whole pipeline with the same physical and mechanic features of the original coating without any weak spots on the pipeline. At present the weak spot of the pipeline is the welding joint, and the renewing systems of the anticorrosive protection for the joints are different and depend, when possible, on the type of coating which has been applied onto the pipe at the factory.
The following systems are used: sealing plastic tapes with cold-application; textile tapes impregnated with various sealing and protective materials such as bitumen, various resins, and so on; powder or liquid resins; thermoretractable plastic sleeves; single- and two-layer coextruded sheathings and tapes, and so on, with suitable known procedures, though substantially manual.
The objective is anyway to give a certain continuity to the anticorrosive protection on the pipe, by means of solutions showing the highest reliability, or better, the same reliability as the protection applied onto the rod.
The present invention relates particularly, but without limitations, to the coating, of the multi-layer type, and preferably also of the three-layer type, in situ, of welding joints and of damaged areas of metal pipes coated at the factory by means of thermoplastic materials, such as for instance PE/PP, according to the DIN specifications DIN 30670 and DIN 30678, and to similar specifications.
A non-limiting plan concerning the stages of the application of the three-layer PE/PP coating in the plant is the following:
blasting of the metal surface of the pipe;
possible washing of the surface with a phosphating agent;
heating of the metal pipe at the required temperature, i.e. 150÷230xc2x0 C.;
application of a powder/liquid epoxy primer with a thickness of 50÷300 xcexcm;
application of the thermoplastic adhesive with a thickness of 200÷400 xcexcm;
application of the outer PP/PE, or others, thermoplastic material by means of extrusion, with the required thickness, usually between 1 and 4 mm.
Such kind of three-layer PP/PE coating is thought to be the best and most reliable anticorrosive coating existing today for underground pipe-lines; it is moreover extremely important to renew the coating of the welding joints and of the damaged areas by means of materials and procedures ensuring the same features of reliability of the original coating.
At present the anticorrosive protection of the joint is renewed in situ in different ways:
A) for metal pipes with a three-layer PE coating for operative temperatures below 70xc2x0 C.
blasting of the metal surface for the whole length of the uncovered joint/pipe.
Heating of the portion of blasted pipe normally by means of electric induction.
Possible manual application of epoxy primer.
Manual positioning on the joint of a thermoretractable sleeve made of PE, consisting of an outer layer in PE and of an inner layer in sealingxe2x80x94adhesive material, in such a length as to exceed of about 10 cm the coating present on the rods.
Manual heating of the sleeve by means of a LPG torch (liquefied petroleum gases), till said sleeve is completely suitable for the underlying pipe, trying to let out absorbed air bubbles by heating and pressing manually the sleeve surface repeatedly so as to remove possible air bubbles.
Plastic tapes or coatings based on epoxy resins, PU (polyurethane), and so on, are used as an alternative to the sleeve.
B) For metal pipes with a three-layer PP coating for operative temperatures below 130xc2x0 C.
As far as PP is concerned there are no thermoretractable sleeves resisting at such high temperatures; therefore, for the coating of welding joints special thermoplastic polypropylene materials are used, said materials being patented for instance by the company Montell, ex Hymont, in powder, sheathing, band and tape form.
The typical procedure followed to carry out the coating includes the following operations:
blasting of the metal surface.
Possible bevel cut of the rim of the surrounding plastic coating.
Heating of the metal surface, preferably by means of electric induction.
Manual application of the liquid or powder primer onto the metal surface, with a brush, a shoes, an electrostatic gun, a flame gun, fluid-bed nozzles possibly mounted and manually operated on a circular template placed around the pipe.
Manual application of adhesive with an electrostatic gun, a flame gun or fluid-bed nozzles possibly mounted and manually operated on a circular template placed around the pipe.
Manual positioning of a two-layer-sheathing/band (with suitable dimensions and after its heating) on the area of the pipe to be coated, said sheathing/band consisting of coextruded PP and adhesive, or manual positioning of a sheathing/band divided between PP and adhesive (as an alternative to the adhesive band it is possible to use powder adhesive).
The sheathing/band is used with or without the overlap of the existing surrounding coating (in case of no overlap the free space is renewed at the end of the application of the sheathing/band by filling it with PP or with its adhesive copolymer by extrusion).
The thickness of the coextruded sheathing/band is preferably the final thickness one wants to obtain without using more layers.
After the sheathing/band is placed onto the tube, said sheathing/band is fixed and pressed onto the tube in different ways by clamping it by means of suitable belts, of thermoretractable tape, of a tubular compressed-air band, of a metal band with one or more sectors, so as to adapt and make it adhere to the substrate.
Possible additional heating of the sheathing/band so as to ensure the complete melting of the adhesive and a suitable melting of the PP.
After the application of the sheathing/band, cooling to the desired temperature.
As an alternative to the sheathing/band, with the same dimension as the area to be coated, it is possible to use a coextruded two-layer tape, made of PP and adhesive, with a thickness preferably below 1 mm, wrapping such tape around the pipe with overlapping of several layers till the required thickness is reached.
Another alternative can be the application, after that of the primer and of the adhesive, of the PP or of the PP copolymer with a flame gun, till the required thickness is reached, overlapping the surrounding PP three-layer coating which has been applied in the plant.
Another common technique is to mount, after the application of the primer and possibly of the adhesive, a ring/mould around the area to be coated, overlapping the surrounding coating area, so as to form a hollow space which is of the same thickness as the desired coating, and to fill said hollow space with the required thermoplastic material by means of an injection press or an extruder.
By means of the aforesaid materials and of the methods used up to now for the application in situ of the anticorrosive coating onto the welding joints and onto the damaged areas of coated pipes with the three-layer procedure it is possible to obtain coatings having acceptable features, though extremely lower than the original coating made at the factory.
The methods used, most of which consist of manual operations depending on the worker""s ability, show several critical points and cannot ensure certain times, operative conditions and repeatability in the results [such as, for instance, the correct positioning of the sheathing/band, or of the tape on the pipe; irregular and inhomogeneous stress and pressure the whole length of the sheathing/band, or tape, often resulting in the entrapment of air when the sheathing/band, or tape touches the uncovered pipe and the overlapping areas; the several overlapping of the tape which must be perfectly impermeable and prevent the xe2x80x9ccurtain-effectxe2x80x9d with subsequent air entrapment; the difficulty in obtaining a perfect adhesion of the material which has been applied by extrusion onto the rims of channels which have been left empty while applying the sheathing/band; possible air entrapment during the implementation of the coating with the spray application of the powder by means of a flame gun].
The main object of the present invention is, therefore, to provide a method for the renewal in situ of the anticorrosive protection of the welding joint and of the damaged coating areas avoiding the disadvantages of known techniques, overcoming their critical aspects and carrying out in practice the same anticorrosive protection made at the factory by using the same materials (powder or liquid primer; adhesive copolymers in powder form or in the form of a ready-to use sheathing/band/tape carried out by extrusion; polyolefin thermoplastic materials in the form of a ready-to use sheathing/band/tape carried out by means of extrusion and in the form of a ready-to use sheathing/band/tape carried out by means of coextrusion with the adhesive copolymer).
Another object of the present invention is to provide a mechanized equipment for the implementation of said method, which turns around the pipe and carries out the various working operations required in pre-determined and repeatable working conditions, resulting in a perfectly reliable coating according to the DIN specifications DIN 30670, DIN 30678 and similar.
A form of embodiment of the present invention provides that the application of the sheathing/band/tape in thermoplastic material (for instance PE/PP and adhesive copolymer, separately carried out by means of extrusion, or for instance PE/PP and adhesive copolymer, carried out in a double layer by means of coextrusion) onto the joint/pipe to be coated is carried out mechanically around the pipe by means of a suitable roll, made for instance of silicone rubber, idle or motorised, during its rotatory-revolutionary movement around the pipe and preferably in touch with the pipe itself.
Another feature of the present invention further provides that the sheathing is preferably placed around the pipe by transposition from the rubber roll around which it had been previously wrapped or by feeding from another auxiliary roll around which it had been previously wrapped.